Switched-capacitor (“SC”) gain amplifiers have found wide applications in analog signal processing, such as automatic gain control (“AGC”), preamplifiers, analog-to-digital converters (“ADC”), and otherwise. A conventional SC gain amplifier 100 is shown in FIG. 1. SC gain amplifier 100 includes capacitors Cf and Cs, switches T1 to T6, and operational amplifier (“OP-AMP”) 105 coupled in an integrator configuration. In FIG. 1, during operational phase 1 (often referred to as the reset or sampling phase), an input signal is sampled and stored on capacitors Cs. In operational phase 2 (amplification phase), the input signal is amplified. With this strategy, the offset and low frequency noise of OP-AMP 105 are stored in Cs during phase 1 and canceled in phase 2. OP-AMP 105 may be a one stage gain boost amplifier or a two-stage amplifier.
The type of OP-AMP 105 used depends on the particular application; however, a two-stage OP-AMP is common due to its relative high speed capability and large output voltage swing. FIG. 2 illustrates a conventional two-stage OP-AMP 200 that may be used to implement OP-AMP 105 in FIG. 1. However, the architecture of two-stage OP-AMP 200 suffers from stability issues. In particular it is difficult to design OP-AMP 200 such that it maintains operational stability during both the reset/sampling phase and the amplification phase.
The stability issues flow from the fact that the feedback factor is equal to one during the reset/sampling phase, but equal to Cf/(Cf+Cs+Cp) during the amplification phase. For example, if OP-AMP 200 is designed for an open loop bandwidth of 100 MHz and a beta= 1/9 during the amplification phase, then OP-AMP 200 will have approximately an open loop bandwidth of 900 MHz during the sampling reset phase. To design such an op-amp with good phase margin stability in both phases is difficult and expensive. The situation is worse if the design of OP-AMP 200 calls for a smaller feedback factor during the amplification phase.
To compensate for these stability issues, conventional approaches include shorting the inputs and outputs of SC gain amplifier 100 to reference voltages during the sampling/reset phase. However, this technique necessitates costly reference circuitry, and the offset voltage of OP-AMP 105 is not cancelled. Additionally, during the sample/reset phase, the virtual ground terminal of OP-AMP 105 can be tied to true ground via a switch to reduce the feedback factor beta. However, this technique is sensitive to switch timing, mismatch, and other issues. In addition, auto-zero techniques, which include relatively costly circuitry, such as reference circuits, may affect the main amplifier performance.